Tunable multicavity type magnetron tube



Jan. 6, 1953 M. A. HERLIN ETAL TUNABLE MULTICAVITY TYPE MAGNETRON TUBE 2 SHEETSSHEET 1 Filed Dec. 10, 1945 I l llllll lllll l ull ula FIG. 3

MELVIN A. HERLlN WILLIAM V. SMITH ATTORNEY Jan. 6, 1953 M. A. HERLIN ET AL 2,624,864

' TUNABLE MULfIICAVITY TYPE MAGNETRON TUBE Filed Dec. 10, 1945 2 SHEETS-SHEET '2 GCDNCO 25222253336666 FIG.4,

' INVENTOR. MELVIN A. HERLIN WILLIAM V. SMITH ATTORNEY Patented Jan. 6, 1.953

UNITED STATES PATENT OFFICE Melvin A. Harlin, Cambridge, and William Y,

S r s, ssisno s b mssns assignments, to the United States of'Americaas sn s nt tl b he taty 0? Wa 5 Claims.

This invitat on rslatss t9 s sctri a app ratu and more particularly to tunable magnetrons.

two of tunable-ma n tr n an sxtst tun ble p v t s ona ris @QQPlfid t one ot th oavi ss o p u al a it ma netron b means i r s nan ape tu e.- If th s avity es nato ss. an a rec able act on the tota ener y store in tho ombinati n o gnetr and e itr resona or, a Qhanss th eso an fre uency f t e cavity reso ato will e lt i s m sma le Qll ill e f the i squsn i he ca i re natormagnetron combination. The stabilization facto is m asurs o the re at ve ener st ages of ssot a-t 1. and ma n ron: Th s abi izatiqn iastor S is. dsflnsd in the foll g ou tion:

Stored energy in resonator and magnetron S and. en y in m g ro a o where AF is the frequency change of the resonator-magnetron combination corresponding to a frequency change A f of the resonator alone;

The addition of a tunable cavity resonator to he m nstmo n. th s fashio s il n ho .i troduction of new and extraneous modes of oscillation in addition to the desired pi-mode of oscillation for the magnetron. The pi-rnode of oscillation in the magnetron is that mode in which h altsrg t an d e m t o min he Walls Q h var ous, indi idual cavities. i h n th plural cavity magnetron) are positive and negative respectively,

One such tunable cavityresonator for tuning nagnetrons has been designed in the past. hi rts sattt stand b fa tits s s ar o th t al sns s o he so -m n ron combination and therefore had a high stabilizationfactor S. The high stabilization factor was accompanied by a strong tendency of the magnetron to operate in one of the aforementioned undesired modes. This tendency was suppressed by discriminating against the. undesired modes with some preferential loading means. The use of ch P a in fis ,.ti s; in

st tha undesired modes, but

. as 2,. M has the tssstsatsgas 9;;- 1

and increased expense.

binst qn the unit o itv is o these ir d mss sot oss oa h.6 onset oi th in en on 'i t provide a tunable. res nat -m n ro t binat n i h. substa ial rang 05 n g a of bs o sr 1 LOW,

A t l further object is to pro e a p s an insxnsnsivs stru ture for accompl he aforementioned objects.

Qthsr obisst a d a vant of n e t n will be appa en du ng the u se o h ollowin desc i t on:

The attainment of the maximum-tuning r a-nge, cons stent wi h fre dom m the n si modes o oscil ation, is :made possible by d nns t e ex erna tun b e c y resonator to have a relatively low energy storage. The cavity resonator is designed (for reasons which will be indicated hereinafter) to have a stabilization factor S (Equation 1) less than 3.

In the accompanying drawings forming a part of this specification and in which like numerals a e employ d to des gnate l ke part throug out the sam Fig. 1 is a broken-away isometric View of a tunable cavity resonator-r-magngfiron combination embodying he prin iples oi the present inen F g. 1A is an isometric view of the -.c avity res,- Onator and l ng aperture of Fig. 1 tea somewhat rsdusedsea Fig. 2 is a sim li i d sestiona to iew oi Fig. 1 to a somewh t rsduasdsoals:

Fi 3 is. the equi al nt. sies ri al. ireuit o Fig. 1, the subscripts 1, 2, and 3 referring to the magnetron, oupl n anemia and .sa-vity reasonator respectively; and

Fig. 4 shows graphs of a. Stabilization factor S as a; function otthe patamsts s ff-1a 3 o id; linscur es) and '1 Rela i e In v 1s wa elen h the ratio strap I3. Anode segments H include therebetween the individual oscillator cavities I4 within magnetron II]. Numeral 23 designates a ring structure continuous with and joining the anode segments I I at their outer terminations. An ex ternal tunable cavity resonator I5 is coupled to one of these magnetron oscillator cavities (i. e. I 8) by means of a substantially c-shaped resonant coupling aperture IS. The aperture It has been shown c-shape (best shown in Fig. 1A), but other equivalent apertures, approximately resonant at the magnetron frequency, would be suitable.

The resonator I5 and coupling aperture I6 of Fig. 1 are shown in Fig. 1A drawn to a somewhat reduced scale. The resonator I5 is constructed to have a relatively small ratio of height h to diameter d. The coupling aperture I6 extends through one end closure of the cavity resonator I5.

Referring to Fig. 2 which shows a simplified sectional top view of the cavity resonator-magnetron combination of Fig. 1, drawn to a somewhat reduced scale, coupling aperture I6 couples into the magnetron cavity I8 at a high current point 20. The cavity resonator I5 has a flexible diaphragm 2I for the end closure opposite the end closure containing the C-shaped aperture I5. Numeral 22 designates a stud attached to the diaphragm 2 I, said stud being adjustable to vary the geometry of the resonator 15 to provide tuning thereof.

The exact size, shape and dimensions of resonator I5 for a given type of magnetron are in general determined empirically. One method is asfollows:

The C-shaped aperture I6 is cut in one end 010. sure of resonator I5, and through the ring structure 23 of magnetron ID to approximate dimensions determined as follows:

I l 4 -A a z c (3) S 0.15m

where Sis the slot width (see Fig. 1A),

A1. is the sum of the areas of the side arms of the C (Fig. 1A)

Ac is the area of the metal facing of gap I9 (i. e. Ac=bt),'where b is the dimension between the side arms of the C (Fig. 1A) and t the slot 'depth (Fig. 2) and A is the operating wavelength.

The exact dimension of the aperture I6 is determined by adjusting the aperture I6 until the separation between the pi-mode and the undesired. modes is greater than the desired tuning range. e

The correctness of the dimensions of cavity resonator I5 is checked as follows:

Tuning curves of cavity resonator I5 alone, indicating A) (Equation 2) as a function of the motion of stud 22, and of the resonator-magnetron combination indicating AF (Equation 1) as a function of the motion of stud 22, are plotted. As will be described in detail hereinafter, it is desirable to have a stabilization factor S of from 1 to 3 with an optimum value of 2. This indicates that Af 0.6( (Equation 2) "If this relation is not satisfied empirically, it is necessary to decrease the relative energystorage of resonator I5, and this may be done by designing it with a somewhat smaller ratio, or, in general, with a. somewhat smaller Q. While the resonator I5 is here shown as having a pill-box like construction (i. e. cylindrical with a small ratio of height to diameter) it is obvious that any equivalent cavity resonator of other shape and dimensions, but having substantially the same energy storage characteristics, may be substituted.

The actual value of S for a given resonatormagnetron combination may be determined from the tuning curves. From Equation 1:

The operational features of the tunable magnetron embodying the present invention are as follows:

The frequency of the plural cavity magnetron I0 may be varied by coupling the external tunable cavity resonator I5 to one of the individual cavities within magnetron ID (i. e. cavity I8) by means of the resonant coupling aperture I6.

The following design features of the cavity resonator I5 and coupling aperture I6 are included to better satisfy the requirement that there be a relatively low storage of energy within resonator l5:

a. Resonator I5 has a relatively small ratio of height to diameter (relatively low Q).

b. The C-shaped aperture I6 is not centered in the end closure of resonator I5 but is almost entirely in a semi-circular portion of the end closure. The C-shaped aperture I6 occupies substantially the maximum current region of the electromagnetic field within resonator I5. The aperture I6 couples into cavity I8 of magnetron I0 at the high current point 20. The current at point 20 will correspond to the maximum ourrent within resonator I5 (the current intensity within resonator I5 decreasing beyond this point) and hence the storage of energy within resonator I5 is minimized.

The coupling aperture I6 is made large to displace the energy of the extraneous undesired modes. This has been the reason for placing the aperture in the end closure of the cylindrical resonator I5 rather than in its side wall.

The end closure of the cavity resonator I5 opposite the end closure having coupling aperture I6 is a movable diaphragm 2|, the position of which is variable by means of the stud 22. The adjustment of stud 22 serves to vary the resonant frequency of the cavity resonator I5 thereby varying the frequency of the radio frequency output of magnetron I0,

The equivalent electrical circuit for the apparatus of Figs. 1, 1A and 2 is shown in Fig. 3.

L is the equivalent inductance, C the equivalent capacitance and Z the characteristic impedance, the subscripts 1-, 2 and 3 referring to the-magnetron IIl, coupling aperture I6 and cavity resonator I 5 respectively.

5 the equivalent circuit of Fig. 3, and are plotted on Fig. 4, curves A, B, A" and B".

I Curve A (Fig. 4) is a graph of g as ordinate vs.

abscissa for =0.01.

The stabilization factor (Equation 1) for the pi-mode (wavelength M) will in general be different from the stabilization factor for the upper and lower modes (wavelengths A and A").

Curve C (Fig. 4) is a plot of the X stabilization Curve D is a plot of the A, A stabilization fac- Curve E is a plot of the A, A stabilization factor S as ordinate vs. i as abscissa for =0.0l.

Curves A, A, B and B show that the mode separation is strongly dependent on the value of factor S as ordinate vs. as abscissa.

tor S as ordinate vs. as abscissa for =O.14.

Curves D and E show that the stabilization factor S is however only slightly dependent on the 2 8.1116 of Z The crossover region (at which the stabilization for the center mode equals that of the two extraneous modes) occurs at an approximate stabilization factor 8:3, over a wide range of values for Operation in a mode other than the desired pimode is generally associated with an excessive storage of pi-mode energy within cavity resonator l5, consequently impeding the build-up of pi-mode oscillations within magnetron It). To minimize this undesired operation, it is necessary that the cavity resonator I have a lower storage of energy at the pi-mode than at the upper and lower extraneous modes. This is equivalent to requiring that the stabilization factor S for the desired pi-mode shall be less than the stabilization factor for the upper and lower undesired modes. The stabilization factor S for the pi-mode must then be less than 3 with an optimum value of 2.

From Equation 1, if S is less than 3, then the stored energy in the resonator is less than twice the stored energy in the magnetron.

Alternatively from Equation 1, if S is less than 3, then the stored energy in the resonator is less than two-thirds the stored energy in the combination of resonator and magnetron.

Because of the small stabilization factor S, the tuning range of resonator I5 is relatively great. A small motion of diaphragm 2| results in a fairly 6 large change in the resonant frequency of resonator [5.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious that various changes and modifications may be made therein without departing from the scope of the invention.

What is claimed is:

1. A tunable magnetron oscillator including a plural cavity magnetron having a plurality of individual oscillator cavities formed therein, a cavity resonator having substantially a cylindrical construction with a relatively small ratio of height to diameter, said resonator having an energy storage less than twice the energy storage of said magnetron, said resonator further having a substantially C-shaped resonant aperture in one end closure, said aperture coupling said cavity resonator to one of said oscillator cavities, 2. diaphragm constituting the other end closure of said resonator and means mounted on the outside of said diaphragm for moving said diaphragm thereby varying the frequency of said cavity resonatormagnetron combination.

2. A substantially cylindrical cavity resonator having a relatively small ratio of height to diameter, said resonator having one end closure having only one approximately resonant aperture therein, a diaphragm constituting the other end closure of said resonator and means for moving said diaphragm thereby tuning said resonator.

3. A substantially cylindrical cavity resonator having a relatively small ratio of height to diameter, said resonator having one end closure with a substantially C-shaped approximately resonant aperture therein, a diaphragm constituting the other end closure of said resonator, and means for moving said diaphragm to thereby tune said resonator.

4. A tunable magnetron oscillator including a plural cavity magnetron; a substantially cylindrical cavity resonator having a relatively small ratio of height to diameter; said resonator being resonant at a frequency in the vicinity of the frequency of oscillation of said magnetron; said resonator having one end closure having only one approximately resonant aperture therein, a diaphragm constituting the other end closure of said resonator and means for moving said diaphragm thereby tuning said resonator; and said resonator being coupled to a cavity of said magnetron through said aperture.

5. A tunable magnetron oscillator according to claim 4, wherein said aperture is positioned at a high current region of said cavity of said magnetron.

MELVIN A. HERLIN. WILLIAM V. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,287,845 Varian et al June 30, 1942 2,404,226 Gurewitsch July 16, 1946 2,408,237 Spencer Sept. 24, 1946 2,414,085 Hartman Jan. 14, 1947 2,445,282 Slater July 13, 1948 2,446,765 Hartman Apr. 12, 1949 2,501,052 Herlin Mar. 21, 1950 

